An electrophotographic photoreceptor comprises a support having an electrically conductive surface and a photosensitive layer formed on the surface. In general, however, a non-photosensitive layer called an undercoat layer or interlayer is disposed between the photosensitive layer and the support for improving adhesion between the photosensitive layer and the support, improving coating applicability of photosensitive-layer formation, protecting the support surface, covering surface defects on the support, protecting the photosensitive layer against electrical breakdown, improving charge injection property into the photosensitive layer, etc. Known materials for use in forming this layer include polyurethanes, polyamides, poly(vinyl alcohol), epoxy ethylene-acrylic acid copolymers, ethylene-vinyl acetate copolymers, casein, methyl cellulose, nitrocellulose, phenolic resins, and organometallic compounds, as described, for example, in JP-A-48-47332 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-51-114132, JP-A-52-42123, JP-A-59-23439, and JP-A-62-284362.)
However, these conventional undercoat layers have the following drawbacks. There are cases where according to the composition of the charge generating layer, the movement of charge carriers to flow into the support is inhibited, to thereby cause re-coupling thereof with countercharge carriers within the charge generating layer or accumulation thereof at the interface between the undercoat layer and the charge generating layer to form a barrier of space charges. Upon repeated use, such a photoreceptor undergoes a decrease in electrification potential, an increase in residual potential, etc. In addition, since charge transport in these conventional undercoat layers is attributable mainly to the water contained therein, the properties of the conventional photoreceptors considerably vary with changing humidity. To eliminate these drawbacks, it has been proposed to incorporate an electron-donor into an undercoat layer. For example, JP-B-61-35551 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses the formation of a barrier layer containing a non-hydrophilic peptide polymer and either an electron-donor or an electron-acceptor, while JP-A-60-218655 discloses the formation of an undercoat layer containing an electron-donor. In JP-A-61-80158 is disclosed the formation of an undercoat layer containing a hydrazone compound. Further, JP-A-61-204640 discloses the formation of an undercoat layer containing a charge transporting material such as imidazole, pyrazoline, thiazole, oxadiazole, oxazole, a hydrazone, a ketazine, an azine, carbazole, polyvinylcarbazole, etc.
In contrast to the above-described technique, a technique for overcoming the above-described drawbacks by incorporating an electron-acceptor into an interlayer to facilitate electron transfer therethrough is disclosed in, e.g., JP-B-61-35551 and JP-A-59-160147. Furthermore, JP-A-58-209751 discloses the formation of a precoat layer containing a n-type dye or pigment, and JP-A-63-210848 discloses the formation of an undercoat layer containing an electron-transferring pigment.
However, photoreceptors having an undercoat layer containing an electron-donor as described above have a problem that the undercoat layer cannot fully perform its function because the electrons generated in the photosensitive layer tend to be trapped and re-couple with positive holes to cause a sensitivity decrease.
On the other hand, in photoreceptors having an undercoat layer containing an electron-acceptor, the undercoat layer sufficiently performs its function. However, since the electron-acceptors disclosed in JP-B-61-35551 and JP-A-59-160147 are soluble in solvents, such photoreceptors have a drawback that during the formation of a photosensitive layer on the undercoat layer by coating, especially by dip coating, the electron-acceptors partly dissolve away and come into the photosensitive layer or the coating solution. In this respect, the pigments disclosed in JP-A-58-209751 and JP-A-63-210848 are slightly soluble or insoluble in solvents and hence do not dissolve into photosensitive layers. However, since the undercoat layer containing this kind of pigment is formed by applying a dispersion of the pigment in a resin which is solvent-soluble, photoreceptors having this undercoat layer have a drawback that during the formation of a photosensitive layer on the undercoat layer by coating, the resin partly dissolves away to cause coating film defects, making the undercoat layer incapable of sufficiently performing its function. The present invention has been achieved under the circumstances described above.